1. Field of the Invention
The present invention relates to a phase retarder film.
2. Description of the Related Art
Phase retarder films have been used as optical compensators for STN liquid crystal display devices and in more recent years their various applications to optical compensators for other types of liquid crystal display device than the STN type have also been studied. For example, there have recently been cases that liquid crystal display devices of the STN and TN types are required to have specific viewing angle characteristics depending upon their application. For this reason there is a need for many kinds of phase retarder films having optical characteristics for respective applications. However, the currently developed phase retarder films are restricted to two types, i.e., one having characteristics close to those of the uniaxially orientated films used for the aforementioned STN liquid crystal display device and the other having characteristics close to those of the perfectly biaxially orientated films comprising a layered inorganic compound layer, in which the in-plane refractive index is different from the refractive index in the direction of thickness, as described in EP-A-0541308. There have not been developed such phase retarder film as having characteristics intermediate between these two types of phase retarder films so far.
Proceedings of Eurodisplay '93, p. 149 reports a simulation where a phase retarder film having refractive indices, n.sub.x =1.618, n.sub.y =1.606, and n.sub.z =1.493 and a thickness of 9.296 .mu.m is effective as optical compensator for the bend orientation type OCB mode liquid crystal display device (.pi. cell). Calculation based on the data for the phase retarder film indicates that the in-plane retardation is 112 nm and the ratio of the in-plane retardation to the retardation normal to the plane is 0.101.
However, the phase retarder films which have heretofore been used for the STN liquid crystal display device have a ratio of the in-plane retardation to the retardation normal to the plane of not less than 2.0 due to the uniaxially orientated structure of the films. On the other hand, the phase retarder films comprising a layered inorganic compound layer having an in-plane refractive index and a refractive index normal to the plane which are different from each other can not be produced unless the in-plane retardation is in the range of 0 to 50 nm as described in EP-A-0541308. Therefore, any phase retarder film having characteristics useful as, for example, optical compensator for the aforementioned .pi. cell can not be obtained from those films which have been developed for current mass-production of phase retarder films or even from their combination. For this reason, there is a need to develop phase retarder films having optical characteristics different from those of the conventional ones. Especially the development of phase retarder films made of light weight thermoplastic resins and a process for producing efficiently and advantageously in industry such phase retarder films.